Devel::Declare can install subroutines called declarators which locally take
over control of Perl's parser, allowing the creation of new syntax, including
keywords and other constructs that might not be, in themselves, valid Perl 5
code.

This module can be used for adding new keywords, completely in Perl code. It
allows a subroutine to be executed during the Perl code compilation, meaning
that syntax can be transformed into equivalent code for Perl on-the-fly.